An important aspect of rocket flight is
stability
and
control
of the rocket.
Model rockets,
stomp rockets,
and
bottle rockets
all use aerodynamic forces to
provide
some measure of flight stability. But these types of toy rockets do not
have any system for flight control.
In order to successfully complete its mission, a
full scale rocket
is designed with systems for both stability and control. The
Guidance system
usually includes sophisticated
sensors and computers to detect the orientation, location, and
speed of the rocket.

To maneuver the rocket in flight, several
different systems
can be used. Early rockets,
and some air-to-air missiles, use movable aerodynamic
surfaces like the elevators
on an airplane. Of course, this system only works on
rockets which remain in the atmosphere.
Later rockets designed to exit the atmosphere used small vanes in the
nozzle exhaust to vector
the thrust. Most modern rockets, like the Space Shuttle and the
Saturn V moon rockets, use a system called gimbaled thrust.

In a gimbaled thrust system, the exhaust
nozzle
of the rocket can be swiveled from side to side. As the nozzle is moved,
the direction of the
thrust
is changed relative to the
center of gravity
of the rocket. On the figure at the top we show three case.
The middle rocket shows the "normal" flight configuration in
which the direction of thrust is along the center line
of the rocket and through the center of gravity of the rocket.
On the rocket at the left, the nozzle has been deflected to the left
and the thrust line is now inclined to the rocket center line
at an angle a called the gimbal angle. Since the thrust
no longer passes through the center of gravity, a
torque
is generated about the center of gravity and the nose of the rocket
turns to the left. If the nozzle is gimbaled back along the center line,
the rocket will move to the left.
On the rocket at the right, the nozzle has been deflected to the right
and the nose is moved to the right.

Here's a computer animation of the motion of the rocket as the
nozzle is gimbaled:

You can also use this Java applet to investigate the motion of
a rocket as the nozzle is gimbaled:

This page shows an interactive Java applet which demonstrates the motion
of a rocket with a gimbaled nozzle

You can change the nozzle angle by using the buttons at the
bottom of the simulator. "Reset" brings the rocket back to its
original configuration.

You can download your own copy of this simulator for use off line. The program
is provided as Gimbal.zip. You must save this file on your hard drive
and "Extract" the necessary files from Gimbal.zip. Click on "Gimbal.html"
to launch your browser and load the program.